Method of sampling from a multiphase fluid mixture, and associated sampling apparatus

ABSTRACT

The invention relates to a method of sampling a fluid phase in a multiphase fluid mixture flowing in a pipe ( 7 ) under given pressure conditions (P), said method comprising the following steps; admitting the multiphase fluid mixture into a phase separator ( 9 ); separating the multiphase fluid mixture so as to isolate the fluid phase for sampling; pressurizing a sampling device ( 10 ) to the given pressure in the pipe ( 7 ); and recovering a sample of said fluid phase in said sampling device. According to the invention, the method further comprises a step of increasing the pressure in the phase separator ( 9 ) in successive stage until said pressure in the separator reaches the give pressure (P) in the pipe ( 7 ). The invention also provides a sampling apparatus for taking a fluid phase from a multiphase fluid mixture flowing in a pipe ( 7 ) under given pressure conditions (P), said apparatus comprising: a phase separator ( 9 ); admission means ( 11, 12   a ) for admitting the multiphase fluid mixture into said separator; and sampling means ( 10 ) for taking a sample of said fluid phase, said sampling means being connected to the phase separator. According to the invention, the apparatus further comprises closure means ( 8   a   , 8   b   , 8   c ) for increasing the pressure inside the separator ( 9 ) in successive stages.

The present invention relates to a method of sampling a fluid phase froma multiphase fluid mixture, and to associated sampling apparatus. Apreferred application of the invention relates to a method of taking asample in order to measure the density of an oil coming from theeffluent of an oil well, and to do so under conditions of temperatureand pressure that correspond to those of the line for enabling saideffluent to be transported.

After a hydrocarbon well has been drilled and made safe, the well is putinto production in order to identify clearly the nature and themagnitude of its components. These tests, commonly referred to as “welltesting”, are particularly important for future working of the oildeposit since they make it possible to estimate its capacity forhydrocarbon production and thus the yield of the well.

It is common practice for these tests to make use of means forseparating the various phases of the effluent (water, oil, and gas).Since the various phases are of different densities, separation isperformed continuously in a separator where the phases settle.Downstream from the separator, the aqueous phase is discarded, possiblyafter additional purification, and the hydrocarbons are burned off,stored, or injected into another pipe. The measurements that areperformed serve to determine the flow rate of the various phases underthe temperature and pressure conditions of the separator. Thereafter, inorder to provide these flow rates under “commercial” well operatingconditions, i.e. under conditions of temperature and pressure close toatmospheric conditions, and in units that are meaningful to the welloperator (typically volume flow rates corresponding to a daily number ofbarrels of oil and a daily volume of gas), it is necessary to makecorrections to “transform” these flow rates as commonly provided bymeasuring instruments.

To provide the most accurate possible data concerning oil productionvolume flow rate from a well, various problems arise. Firstly, it isnecessary to manage to take a sample of oil that is pure, i.e. that isnot “polluted” by the other phases. This is particularly difficult whenusing outlet separators in which the water content of the oil phase canbe as much as 30%, e.g. the separator that constitutes the subjectmatter of French patent application No. 00/05666 filed May 3, 2000 inthe name of the Applicant. Another problem lies in the difficulty intaking the oil sample under the temperature and pressure conditions ofthe separator. Since the pressure conditions of the separator are higherthan those of the atmosphere, care must be taken to take an oil phasesample without decompressing it since that would lead to an unusablesample containing “dead” oil (i.e. oil deprived of its gas).

An object of the invention is to remedy those drawbacks by proposing asampling method for collecting a fluid phase from a multiphase fluidmixture without changing the thermodynamic conditions of the mixture.

To this end, the invention provides a method of sampling a fluid phasein a multiphase fluid mixture flowing in a pipe under given pressureconditions, said method comprising the following steps:

-   -   admitting the multiphase fluid mixture into a phase separator;    -   separating the multiphase fluid mixture so as to isolate the        fluid phase for sampling;    -   pressurizing a sampling device to the given pressure in the        pipe; and    -   recovering a sample of said fluid phase in said sampling device.

According to the invention, the method further comprises a step ofincreasing the pressure in the phase separator in successive stagesuntil said pressure in the separator reaches the give pressure in thepipe.

In the invention, the step of increasing of pressure by successivestages is performed simultaneously with the step of admitting themultiphase fluid mixture into the separator.

In this way, the method of the invention makes it possible to collect afluid phase without changing thermodynamic conditions between the pipeand the sampling device. The method is thus particularly suitable whenthe sample collected in this way is for use in performing accuratemeasurements concerning the mixture as it is when flowing along thepipe, and not as it is under “artificial” operating conditions.Furthermore, since the separation step takes place in a separator thatis not subjected to the flow rate conditions of the pipe, separationtimes can be considerably lengthened, thereby guaranteeing the purity ofthe sampled fluid phase. Thus, by maintaining the same pressureconditions all along the method, from the multiphase mixture of fluidsto the fluid phase which is to be sampled, it is possible to use saidmethod for recovering a pure oil phase sample from an effluent comingfrom an oil well, and to do so without falsifying the measurements takenon the sample, which sample does not contain any “dead” oil specificallybecause of the constant pressure. The method of the invention is thusparticularly effective during well testing operations of the kindmentioned above or for any sampling from a pipe that is exporting aneffluent.

In an advantageous implementation of the invention, the phase separatorcomprises a separator in which the phases settle under gravity and thesampling device comprises a sampling flask and admission means, saidadmission means being connected to the phase separator. In this example,the admission means of the sampling flask are connected to the phaseseparator in such a manner that said admission means are situatedsubstantially in the middle of the level of the fluid phase for samplinginside said separator.

Firstly, the fact of using a separator in which the phases settle undergravity is a solution which is particularly simple and low in cost toimplement; gravity separators are also robust, which means they can beused under conditions as severe as those which apply to testingeffluents coming from oil wells. Secondly, the position of the admissionmeans to the sampling flask guarantees that a pure fluid phase isrecovered since this position is remote from the transition zones withthe other phases of the multiphase fluid mixture within the separatorafter settling.

In a particularly advantageous implementation of the invention, themultiphase fluid mixture is a mixture comprising a majority hydrocarbonphase taken from a first separation step implementing a first phaseseparator on a multiphase effluent coming from an oil well, and the pipeis a pipe for recovering said mixture comprising said majorityhydrocarbon phase under pressure conditions that corresponds to thosewhich exist in said first phase sample.

The invention also provides sampling apparatus for taking a fluid phasefrom a multiphase fluid mixture flowing in a pipe under given pressureconditions, said apparatus comprising:

-   -   a phase separator;    -   admission means for admitting the multiphase fluid mixture into        said separator; and    -   sampling means for taking a sample of said fluid phase, said        sampling means being connected to the phase separator.

According to the invention, said admission means also comprise closuremeans for increasing the pressure in the separator in successive stages.

Other advantages and characteristics of the invention can be seen fromthe following description, given by way of example and with reference tothe accompanying drawing, in which:

FIG. 1 is a diagrammatic view of an application of sampling apparatus ofthe invention; and

FIG. 2 shows another application of sampling apparatus of the invention.

FIG. 1 shows an application of sampling apparatus of the invention. Amain duct 1 receives an effluent from an oil well (not shown) to deliverit into a first gravity phase-separator 2. In the example shown, theseparator 2 is a gravity phase-separator of the kind described in patentapplication No. 00/05666 filed on May 3, 2000 in the name of theApplicant. The separator provides first separation of the components ofthe effluent, i.e. its water, gas, and oil. As shown in FIG. 1, an oilrecovery pipe 3 extends from substantially the middle of the oil phaseinside the separator 2 to a “oil” circuit given overall reference 4.Depending on the type of first separator 2 that is used, the fluidmixture from the duct 3 flowing along the oil circuit can contain agreater or lesser percentage of water. In particular, when using aseparator of the kind described in French patent application No.00/05666, this mixture can contain gas, and its water content can be asmuch as 30%.

Head loss is implemented in the oil circuit 4 in the form of a levelcontrol valve 5 which serves to regulate the level of the multiphasefluid mixture in the first separator 2. The head loss caused by thecontrol valve 5 also allows mixture from the first separator 2 to passinto a sampling device 6 of the invention, and shown in greater detailin the other application of FIG. 2. In FIG. 2, the fluid mixturecirculates in any kind of pipe 7, possibly connected to a duct such asthe duct 3 at the outlet from a separator 2. The sampling device ispreferably connected in parallel with the pipe 7, but it could also beinstalled as a branch connection. Control valves 8 a, 8 b, and 8 cgovern the passage through the sampling device of the multiphase fluidmixture flowing along the pipe 7.

As shown in FIG. 2, the sampling device of the invention comprises aseparator 9 which in this embodiment is a separator in which the phasessettle under gravity. The sampling device 6 also comprises a samplingflask 10 which is connected via a connection valve 11 to the secondaryseparator 9 substantially halfway up said separator and more generallyhalfway up relative to the minimum height of the oil phase expected atthe end of settling in the separator 9. This precaution makes itpossible to be sure that the sample of the oil phase collected in thesampling flask is taken precisely from the desired phase, therebyguaranteeing a sample that is particularly pure. As a function of theway in which the phases are distributed in the multiphase fluid mixture,the position of the connection valve 11 along the separator 9 can bemodified. The ends of the sampling flask 10 are provided with isolationvalves 12 a and 12 b and with a gas evacuation valve 12 c. Thus, byclosing the valves 12 a, 12 b, and 12 c, and by disconnecting theconnection valve 11, it is possible to remove the sampling flask fromthe sampling apparatus of the invention in order to performmeasurements. In particular, in order to discover the density of the oilphase, the set of valves 12 a, 12 b, and 12 c (advantageously the valve12 c can be removable so as to further lighten the assembly) and thefull sampling flask 10 is weighed. In order to ensure that themeasurement is accurate, the weight of the sampling flask and of itsisolation and evacuation valves is preferably as light as possible.Thereafter, in order to ensure that very pure oil is taken into theflask 10, it is advantageous to select a separator 9 that is much largerthan said flask. Satisfactory results have been obtained with aseparator whose capacity is 3 liters associated with a sampling flaskwhose capacity is 0.05 liters.

The sampling method of the invention is described below with referenceto the above-described sampling apparatus. In general, the object ofthis method is to collect a sample of a pure fluid phase in a samplingflask 10 from a multiphase fluid mixture, e.g. effluent from an oilwell. The collected sample should be under the same conditions oftemperature and pressure as those which apply to the effluent flowing inthe pipe 7. In the method, the ratio between the phases of the fluidmixture remains substantially stable over time, thus making it possibleto dimension the sampling apparatus suitably, and in particular the sizeof the separator 9 and the position of the sampling flask 10.

In the application shown in FIG. 1, the effluent from the oil well isbrought under given conditions of temperature T and pressure P into thefirst separator 2 where these conditions of temperature and pressure arenot modified. A first separation step then follows in conventionalmanner after which the duct 3 is used to recover a first fluid mixturecomprising a majority hydrocarbon phase still at the same conditions ofpressure and temperature. After this first step, the fluid mixture whosephase ratio is substantially stable is directed to the oil circuit 4 bythe pipe 7 at a rate which depends on the level control valve 5. Thesampling method of the invention is then used to collect this mixture inthe separator 9 while maintaining the same conditions of temperature Tand pressure P in the pipe 7. Should these conditions vary, particularlyshould the pressure in the secondary separator 9 be lower than thepressure in the pipe 7, then the primary mixture would depressurize andthe resulting oil phase would be “dead” oil that is not suitable fordetermining the production capacity of the oil well.

The sampling flask is initially connected to the secondary separator viathe connection valves 11. The sampling flask is previously brought tothe temperature and pressure T and P, and its isolation valves 12 a, 12b, and 12 c closed. To pressurize the sampling flask, it can very simplybe connected via the valve 12 a to the gas outlet from the primaryseparator, with the gas being allowed to flow by opening the valves 12 band 12 c a little and then closing these two valves. The flask is filledwith gas under the conditions T and P. It is also possible to useexternal compressor means. Once the sampling flask 10 has beenconnected, the valves 8 a, 8 b, and 8 c are opened to allow the primaryfluid mixture from the pipe 7 to flow through the separator 9.Thereafter action is taken on the control valve 8 b to close itprogressively so as to raise the pressure in the separator 9 in stages.In this way, the first quantity of fluid mixture will depressurizebecause the valves 8 a, 8 b, and 8 c are fully open and the volumeinside the separator is empty. Thereafter, this first quantity flowsthrough the separator and by closing the valve 8 b a little, a newquantity of mixture as admitted thereto will depressurize less becausethe pressure inside the separator 9 has increased. In addition, this newquantity of mixture will serve to evacuate a fraction of the initialdead oil that has formed. By proceeding in this way, closing valves insuccessive stages, and thus increasing pressure successively, theseparator is eventually filled with a multiphase fluid mixture whichloses less and less pressure and which progressively expels smaller andsmaller quantities of dead oil. Once the control valve 8 b has beencompletely closed, the secondary separator is full of fluid mixture,comprising a majority hydrocarbon phase, and under the appropriateconditions of temperature and pressure, T and P.

The fluid mixture is then allowed to settle in the secondary separator.After a certain length of time has elapsed, this mixture will have splitinto three phases: a gas phase at the top of the separator, an oil phasein the middle, and a water phase in the bottom of the separator. The oilphase is then particularly pure and suitable for measuring densityand/or shrinkage since the time required for settling has not beeninfluenced by the flow of mixture in the pipe 7, so settling time can berelatively long. After this rest time, the connection valve 11 is openedslowly together with the isolation valve 12 a so as to connect thesampling flask 10 to the oil phase in the separator 9. The oil phase isstill at the given conditions T and P. The isolation valve 12 b is thenopened and the gas evacuation valve 12 c is opened very slightly (by wayof example, this valve can be a precision, needle valve) so that the oilphase penetrating into the sampling flask can expel the gas. In thisway, the flask is filled with oil that is very pure and that is at thesame pressure conditions as the pipe 7 (which are likewise the same asthose in the first separator 2, in the application shown in FIG. 1).

As soon as the first drop of oil appears at the outlet from the samplingflask, the valves 12 b and 12 c are closed. It is then possible to closethe valve 12 a and the valve 11, and to open a purge valve (not shown)between these two valves, thus making it possible to recover thesampling flask filled with oil phase at appropriate pressure andtemperature conditions. It is then possible, for example, to measure thedensity of the oil under the conditions that exist in the pipe 7. To dothis, it suffices to weigh the full flask (after cleaning its ends toremove any oil that has not come from inside the flask). Since the flaskwas previously weighed when empty, the density ρ_(o) ^(P,T) of the oilin grams per cubic centimeter (g/cm³) under the conditions P and T isthus given by:

$\rho_{o}^{P,T} = \frac{M_{o}}{V_{f\; l\; a}}$where M_(o) in grams is the mass of the oil and V_(fla) in cm³ is the(calibrated) volume of the flask.

The oil can then be depressurized by opening the valve 12 b, with thegas contained in this oil phase escaping so as to obtain “dead” oilunder atmospheric conditions of temperature and pressure. By taking theratio of the volumes of oil at P and T and at P_(atm) ad T_(atm), it ispossible to calculate the shrinkage Srkg of the oil as follows:

${Srkg} = \frac{V_{o}^{P_{atm},T_{atm}}}{V_{o}^{P,T}}$where V_(o) ^(P) ^(atm) ^(T) ^(atm) is the volume of oil collected underatmospheric conditions and V_(o) ^(P,T)=V_(fla) is the volume of oilunder the conditions in the pipe 7. The shrinkage and the density of theoil under the temperature and pressure conditions in the pipe 7 thenmake it possible to determine the volume flow rate of the oil underatmospheric conditions on the basis of its mass flow rate (as measuredby instruments, not shown, installed in the pipe 7).

It is also possible to calculate this shrinkage accurately usingmeasurements of oil mass after depressurization, M′_(o), and of oildensity under atmospheric conditions (using a density meter), ρ_(o) ^(P)^(atm) ^(,T) ^(atm) . The shrinkage is then given by the followingrelationship:

${Srkg} = {\frac{V_{atm}}{V_{f\; l\; a}} = \frac{M_{o}}{\rho_{o}^{P_{atm},T_{atm}} \times V_{f\; l\; a}}}$where V_(fla) is the volume of oil collected in the sampling flask.

The sampling method and the associated apparatus of the invention thusmake it very simple and reliable to correct a measurement performedunder certain thermodynamic conditions P and T on effluent from an oilwell so as to obtain a usable result concerning the productionperspectives of said well under “standard” thermodynamic conditionsclose to atmospheric conditions. This method is particularly effectivewhen there is a first separator 2 upstream from the pipe 7 with theoutlet from the first separator providing a fluid mixture that containsa percentage of water that is not negligible but that is substantiallystable. In addition, the method and the apparatus of the invention makeit possible to sample any effluent-exporting pipe under thermodynamicconditions which is an essential factor in ensuring that calculationsperformed on the recovered sample are pertinent.

1. A method of sampling a fluid phase in a multiphase fluid mixtureflowing in a pipe under given pressure conditions, said methodcomprising the following steps: admitting the multiphase fluid mixtureinto a phase separator; separating the multiphase fluid mixture so as toisolate the fluid phase for sampling; pressurizing a sampling device tothe given pressure in the pipe; recovering a sample of said fluid phasein said sampling device; and increasing the pressure in the phaseseparator in successive stages until said pressure in the separatorreaches the give pressure in the pipe.
 2. A method according to claim 1,wherein the step of increasing of pressure by successive stages isperformed simultaneously with the step of admitting the multiphase fluidmixture into the separator.
 3. A method according to claim 1, whereinthe phase separator comprises a separator in which the phases settleunder gravity.
 4. A sampling method according to claim 1, wherein thesampling device comprises a sampling flask and admission valves, saidadmission valves being connected to the phase separator.
 5. A samplingmethod according to claim 1, wherein the distribution of phases in themultiphase fluid mixture flowing in the pipe at the given pressure issubstantially stable over time.
 6. A sampling method according to claim1, wherein the sampling device comprises a sampling flask and admissionvalves, said admission valves being connected to the phase separator,wherein the distribution of phases in the multiphase fluid mixtureflowing in the pipe at the given pressure is substantially stable overtime and wherein the admission valves of the sampling flask areconnected to the phase separator in such a manner that said admissionvalves lie substantially in the middle of the level of the fluid phasefor sampling inside said separator.
 7. A method according to claim 1,wherein the multiphase fluid mixture is a mixture comprising a majorityhydrocarbon phase taken from a first separation step implementing afirst phase separator on a multiphase effluent coming from an oil well,and the pipe is a pipe for recovering said mixture comprising saidmajority hydrocarbon phase under pressure conditions that corresponds tothose which exist in said first phase sample.
 8. Sampling apparatus fortaking a fluid phase from a multiphase fluid mixture flowing in a pipeunder given pressure conditions, said apparatus comprising: a phaseseparator; admission valves for admitting the multiphase fluid mixtureinto said separator; removable sampling device for taking a sample ofsaid fluid phase in said phase separator; and closure valves forincreasing the pressure inside the separator in successive stages andwherein the distribution of phases in the multiphase fluid mixture issubstantially stable over time and wherein the sampling device comprisesa sampling flask connected to the separator via admission valves suchthat said valves are situated substantially in the middle of the levelof the fluid phase for sampling inside said separator.
 9. Samplingapparatus according to claim 8, wherein the phase separator comprises aseparator in which the phases settle under gravity.
 10. Apparatusaccording to claim 9, wherein the multiphase fluid mixture is a mixturecomprising a majority hydrocarbon phase coming from a first separationstep implemented by means of a first phase separator acting on amultiphase effluent coming from an oil well, and the pipe is a pipe forrecovering said mixture comprising said majority hydrocarbon phase underconditions of pressure that correspond to those that exist inside saidfirst phase separator.
 11. Apparatus according to claim 8 wherein themultiphase fluid mixture is a mixture comprising a majority hydrocarbonphase coming from a first separation step implemented by means of afirst phase separator acting on a multiphase effluent coming from an oilwell, and the pipe is a pipe for recovering said mixture comprising saidmajority hydrocarbon phase under conditions of pressure that correspondto those that exist inside said first phase separator.
 12. A methodaccording to claim 2, wherein the phase separator comprises a separatorin which the phases settle under gravity.
 13. A sampling methodaccording to claim 3, wherein the sampling device comprises a samplingflask and admission valves, said admission valves being connected to thephase separator.
 14. A sampling method according to claim 13, whereinthe distribution of phases in the multiphase fluid mixture flowing inthe pipe at the given pressure is substantially stable over time.